Computer simulation has become an indispensable tool in a wide variety of technical endeavors ranging from the design of aircraft, automobiles, and communications networks to the analysis of biological systems, socioeconomic trends, and plate tectonics. In the field of integrated circuit fabrication, computer simulation has become increasingly important as circuit line widths continue to shrink well below the wavelength of light. In particular, the optical projection of circuit patterns onto semiconductor wafers, during a process known as photolithography, becomes increasingly complicated to predict as pattern sizes shrink well below the wavelength of the light that is used to project the pattern. Historically, when circuit line widths were larger than the light wavelength, a desired circuit pattern was directly written to an optical mask, the mask was illuminated and projected toward the wafer, the circuit pattern was faithfully recorded in a layer of photoresist on the wafer, and, after chemical processing, the circuit pattern was faithfully realized in physical form on the wafer. However, for sub-wavelength circuit line widths, it becomes necessary to “correct” or pre-compensate the mask pattern in order for the desired circuit pattern to be properly recorded into the photoresist layer and/or for the proper physical form of the circuit pattern to appear on the wafer after chemical processing. Unfortunately, the required “corrections” or pre-compensations are themselves difficult to refine and, although there are some basic pre-compensation rules that a human designer can start with, the pre-compensation process is usually iterative (i.e., trial and error) and pattern-specific to the particular desired circuit pattern.
Because human refinement and physical trial-and-error quickly become prohibitively expensive, optical proximity correction (OPC) software tools have been developed that automate the process of pre-compensating a desired circuit pattern before it is physically written onto a mask. Starting with the known, desired circuit pattern, an initial mask design is generated using a set of pre-compensation rules. For the initial mask design, together with a set of process conditions for an actual photolithographic processing system (e.g., a set of illumination/projection conditions for a “stepper” and a set of chemical processing conditions), a simulation is performed that generates a simulated image of the pattern that would appear on the wafer after the photoresist was exposed and chemically processed. The simulated image is compared to the desired circuit pattern, and deviations from the desired circuit pattern are determined. The mask design is then modified based on the deviations, and the simulation is repeated for the modified mask design. Deviations from the desired circuit pattern are again determined, and so on, the mask design being iteratively modified until the simulated image agrees with the desired circuit pattern to within an acceptable tolerance. The accuracy of the simulated image is, of course, crucial in obtaining OPC-generated mask designs that lead to acceptable results in the actual production stepper machines and chemical processing systems of the actual integrated circuit fabrication environments.
Photolithographic process simulation generally comprises optical exposure simulation and chemical processing simulation. During optical exposure simulation, the operation of a photolithographic processing system (e.g., stepper) is simulated to compute an optical intensity pattern in the photoresist. The computed optical intensity can be purely two-dimensional, i.e., a function of two variables that treats the photoresist layer as a single plane. Alternatively, the optical exposure simulation can treat the photoresist layer as a volume, and compute two-dimensional optical intensities for a plurality of planar levels within the photoresist volume. In still another alternative, the optical exposure simulation can provide a three-dimensional optical intensity.
During chemical processing simulation, the effects of chemical processing (which may include, for example, treating the exposed/unexposed resist, washing away the exposed/unexposed resist, wet or dry etching, etc.) are simulated to compute a resultant intensity pattern in the resist and/or the wafer. In some cases the chemical processing simulation is carried out in a very simple manner, such as by simply thresholding a purely two-dimensional optical intensity pattern by a fixed, predetermined value to generate a resultant intensity that is a binary or continuous pattern. In other cases, the chemical processing simulation can be more complex by processing the optical intensity pattern in location-dependent, pattern-dependent, or depth-dependent manners, or in other ways. In still other cases, such as during the design or troubleshooting of the stepper machines themselves by a stepper machine manufacturer, the chemical processing simulation is omitted altogether (i.e., is a null step in the photolithographic process simulation), with the two-dimensional or three-dimensional optical intensity pattern itself being of primary interest to the user.
For purposes of clarity and not by way of limitation, the output of an optical exposure simulation is referenced herein as an optical intensity. As used herein, resultant intensity refers to the output of an overall photolithographic process simulation. In cases where the chemical processing simulation is a null step in the overall photolithographic process simulation, the resultant intensity corresponds to an optical intensity. Depending on the particular need, the resultant intensity can represent any of a variety of different images or physical quantities including, but not limited to, an optical intensity, an aerial image, a latent image in a resist film, a developed resist pattern, a final semiconductor pattern, and an etch depth profile within a final semiconductor.
It is crucial to accurately compute the optical intensity pattern in the photoresist and to accurately compute the chemical processing effects in order to enable the overall photolithographic process simulation result to be accurate. In addition to the OPC software context, accurate photolithographic process simulation is also useful in other contexts including, but not limited to, resolution enhancement techniques (RETs) additional to OPCs, in which the effectiveness of a particular RET strategy can be verified by simulation.
One or more issues arise in relation to the simulation of a photolithographic process, at least one of which is resolved by at least one of the preferred embodiments described herein. It is to be appreciated, however, that the scope of the preferred embodiments is also widely applicable to a variety of different optical exposure processes, with or without subsequent chemical processing steps, and is not limited to the particular environment of integrated circuit fabrication. One key issue relates to computation time. For example, as indicated in US2005/0015233A1, which is incorporated by reference herein, computation of a single optical intensity for a single resist plane and a single set of process conditions using conventional methods can take hours or even days. In practice, it is often desired to compute the optical intensity for many different values of one or more exposure system variations, and/or to compute the optical intensity for many different resist planes. It is further often desired to compute the resultant intensity for many different values of one or more exposure system or chemical processing system variations, and/or to compute the resultant intensity for many different planes within the photoresist layer or the processed wafer. This can drastically increase the already-substantial computation time in order to compute the desired overall set of results.
Another issue that arises in photolithographic process simulation relates to properly simulating the effects of interactions between the optical mask itself and the exposure light that is passing through the optical mask. As known in the art, an optical mask (also termed a photolithographic mask or photomask) typically comprises an optically transparent substrate supporting an opaque material layer into which patterns of voids (or patterns of less-opaque materials) are formed. Although the opaque material layer is indeed very thin, it generally needs to have a thickness on the order of the wavelength of the exposure light for sufficient absorptance. However, as line widths continue to shrink below the wavelength of the exposure light, the voids (or patterns of less-opaque material) assume increasingly higher aspect ratios, reminiscent of canyons that get narrower and narrower while their canyon walls remain the same height. In turn, this affects the manner in which the optical mask responds to exposure light wavefronts arriving from off-normal angles, and this varying response can be a source of error for simulations that do not properly not take this effect into account. Other issues arise as would be apparent to one skilled in the art upon reading the present disclosure.